Re: [RESEND PATCH] Makeconfig: Set pie-ccflag to -fPIE by default

[Richard Henderson <richard.henderson@linaro.org>]

On 8/26/22 05:32, Adhemerval Zanella Netto wrote:
> 
> 
> On 25/08/22 18:01, Richard Henderson via Libc-alpha wrote:
>> [ I never saw this arrive in the list archives.
>>    Trying again with my linaro address subscribed. ]
>>
>> We should default to the larger code model, in order to support
>> larger applications built with -static -pie.  This should be
>> consistent with pic-ccflag, which defaults to -fPIC.
>>
>> Remove the now redundant override from sysdeps/sparc/Makefile.
>> Note that -fno-pie and -fno-PIE have the same effect.
>>
>> ---
>>
>> In the case of AArch64, the code changes are small but significant:
>>
>>   0000000000000000 <__libc_init_first>:
>>      0:  a9bd7bfd        stp     x29, x30, [sp, #-48]!
>> -   4:  90000003        adrp    x3, 0 <_GLOBAL_OFFSET_TABLE_>
>> -                       4: R_AARCH64_ADR_PREL_PG_HI21   _GLOBAL_OFFSET_TABLE_
>> +   4:  90000003        adrp    x3, 0 <__environ>
>> +                       4: R_AARCH64_ADR_GOT_PAGE       __environ
>>      8:  90000004        adrp    x4, 0 <__libc_init_first>
>>                          8: R_AARCH64_ADR_PREL_PG_HI21   .bss
>>      c:  910003fd        mov     x29, sp
>>     10:  f9400063        ldr     x3, [x3]
>> -                       10: R_AARCH64_LD64_GOTPAGE_LO15 __environ
>> +                       10: R_AARCH64_LD64_GOT_LO12_NC  __environ
>>
>> In the small model, we are constrained to 15 bits of GOT, with a
>> single shared base, _GLOBAL_OFFSET_TABLE_.  In the large model,
>> each symbol has a page + offset pair.  For small functions like
>> this, where there are no other variable references to share the
>> common GOT pointer, there is no actual code change.
>>
>> r~
> 
> I recall that powerpc wanted to use -mcmodel=large, but it would require to
> adjust a lot of assembly implementations.  Are we sure this change does not
> trigger any regression with current glibc code?

Ah, the resend is missing some detail I included in the original:

> For s390x, I would expect changes similar to c64a10e54441, and
> an eventual similar bug report if this were left unchanged.
> 
> For x86, ppc64 and riscv64, -fpie & -fPIE are identical.

For ppc64 specifically, I audited gcc/config/rs6000/ for references to flag_pic, and the 
only changes between -fpic and -fPIC are for 32-bit ABI_V4.

More generally, our assembly is already set up for -fPIC, which is the pic-ccflag default; 
the difference for -fPIE is only in the selection of external vs local binding.

For aarch64, I've audited uses of the adrp instruction in sysdeps/, and they are what I'd 
expect, e.g.

aarch64/start.S:        adrp    x0, :got:main


and there are no instances of '_GLOBAL_OFFSET_TABLE_', which is indicative of -fpic.

For s390, I've similarly audited uses of the larl instruction:

s390/s390-64/start.S:   larl    %r2,main@GOTENT         # load pointer to main




r~